One lock table and improved iam policies

This version breaks backwards compatibility:

• one dynamodb table is sufficient (there is no advantage to one table per stack since the backend lock mechanism uses the backend key as lock key, rather than the dyname table name) -- ie one table can have multiple locks.
• IAM policy names changed to accomodate better breakdown of policies, and provide policy for whole stacks and individual modules

To update your backend states:

1. remove all the backend.tf files produced by this module
2. run terraform init in each folder that had the backend.tf, to move the state back to local
3. run terraform apply on this module to regenerate all backend.tf
4. run terraform init in each folder that has a new backend.tf, to move the state back to S3